The invention relates to a circuit arrangement for feeding a lamp, which is equipped with
input terminals that are to be connected to a supply voltage source,
a power circuit coupled to the input terminals and provided with at least one switching element for generating a lamp current at a frequency f from a supply voltage supplied by the supply voltage source,
a drive circuit coupled to a control electrode of the switching element for generating a drive signal for rendering the switching element alternately conducting and non-conducting at a frequency f,
a control circuit that comprises a microprocessor and is coupled to the power circuit and to the drive circuit for controlling a lamp quantity,
a protection circuit coupled to the power circuit and to the drive circuit for blocking the drive of the switching element.
Such a circuit arrangement is known. The lamp quantity that is controlled may be, for example, the wattage of the lamp or the lamp current. Such a lamp quantity is frequently controlled by setting the frequency and/or the duty cycle of the drive signal. The protection circuit is intended to preclude damage to the circuit arrangement and/or the lamp caused by high voltages and/or currents. Such high voltages and currents may occur, for example, if no lamp is connected to the circuit arrangement, or if the connected lamp is defective. The protection circuit precludes such damage by rendering the circuit arrangement non-conducting if such high voltages and currents occur. As a result, the power circuit stops generating these high voltages and currents substantially immediately. Consequently, the circuit arrangement is effectively protected.
A drawback of the known circuit arrangement resides, however, in that after the power circuit has stopped operating, the microprocessor in the control circuit continues operating for some time. For example, it is possible that the operation of the power circuit is interrupted by the protection circuit for a short period of time only because the amplitudes of currents and voltages have decreased to such extent within this short period of time that a further interruption is not necessary. During the period of time that the operation of the power circuit is interrupted, however, the actual value of the lamp quantity controlled by the control circuit changes dramatically, so that the control circuit also dramatically changes the duty cycle and/or frequency of the drive signal. For this reason, for example, the power supplied to the lamp immediately after the protection circuit has stopped blocking the drive of the switching element may be much too high. This results in unstable operation of the lamp causing, for example, flickering etc. Flickering may also be caused by the fact that the lamp goes out as a result of the short interruption of the power circuit by the protection circuit, and subsequently goes on again. In principle, this problem could be circumvented if the protection circuit, instead of directly blocking the drive of the switching element, would order the microprocessor to stop, after which said microprocessor stops controlling the lamp quantity and, subsequently, also renders the drive circuit inoperative. Stopping the microprocessor and hence also rendering the drive circuit inoperative, however, takes so much time that the high currents and voltages in the power circuit would already have caused damage by the time the operation of the power circuit is interrupted.
It is an object of the invention to provide a circuit arrangement for feeding a lamp, wherein the circuit arrangement is effectively protected against high currents and voltages in the power circuit by means of a protection circuit, and wherein the operation of this protection circuit does not give rise to unstable operation of the lamp.
To achieve this, a circuit arrangement of the type mentioned in the opening paragraph is characterized in that the protection circuit is additionally coupled to the control circuit for interrupting the operation of the control circuit via the microprocessor, and in that the protection circuit blocks the drive of the switching element until the operation of the control circuit is interrupted. It is to be noted that the protection circuit and the control circuit can alternatively be coupled via the drive circuit.
In a circuit arrangement in accordance with the invention, if high currents and high voltages occur in the power circuit, the operation of said power circuit is interrupted via the drive circuit so rapidly that the high voltages and currents cannot cause damage to the circuit arrangement and/or the lamp. In addition, the operation of the control circuit is interrupted via the microprocessor in that the software of the microprocessor renders the control loop inoperative. The operation of the power circuit remains interrupted until the operation of the control circuit is interrupted. This can be achieved in various ways, for example in that the protection circuit starts a timer, and the drive of the switching element remains interrupted until after a time interval determined by the timer has elapsed. Alternatively, it is possible, for example, to make the protection circuit change a memory content from a first value to a second value, and make the microprocessor reverse this change after the interruption of the operation of the control loop. In the latter method, the drive of the switching element is interrupted for as long as the second value is present in the memory. As the protection circuit simultaneously initiates the interruption of the operation of the control circuit and the interruption of the operation of the power circuit, unstable operation of the lamp is precluded. In a circuit arrangement as described in the opening paragraph, the microprocessor frequently not only plays a part during stationary lamp operation in controlling the lamp quantity, but also enables, prior to this stationary lamp operation, the lamp electrodes to be pre-heated and, subsequently, an ignition voltage to be generated. These different functions of the microprocessor are carried out by different software. In such a case, after the software responsible for forming the control loop has been rendered inoperative and, if, for example, it has been detected that a new lamp is connected to the power circuit, the microprocessor is capable of making sure, by means of different software, that these different stages in the operation are successively carried out.
Satisfactory results were obtained using embodiments of a circuit arrangement in accordance with the invention wherein the lamp quantity is selected among the group formed by the lamp current and the lamp power.
It was found to be advantageous to cause the operation of the microprocessor to be interrupted by the protection circuit through the generation of an interrupt. It is alternatively possible to cause the value of a status bit to be changed by the protection circuit, so that the operation of the microprocessor is interrupted.
It has been found that the protection circuit can block the drive of the switching element in a comparatively simple way by interrupting the connection between the drive circuit and the control electrode of the switching element.
It has also been found that the drive circuit and the control circuit advantageously form part of an integrated circuit provided with a stop-pin and, advantageously, the output of the protection circuit is coupled via this stop-pin to the microprocessor and to the drive circuit.